The invention relates to the metallurgy of iron and steel, and more precisely to the manufacture of parts made from steel which can in particular be used in mechanical construction and shaped by the process known as “thixoforging”.
Thixoforging belongs to the category of processes for shaping metals in the semi-solid state.
This process consists of producing a substantial deformation on a billet heated between the solidus and the liquidus. The steels used for this process are those which are conventionally used for hot-forging, and which are if necessary previously subjected to a metallurgical operation consisting of globulising the primary structure which is conventionally dendritic. In fact, this dendritic primary structure is not adapted to the thixoforging operations. In the course of heating up to temperatures between the solidus and the liquidus, the micro-segregation existing between the dendrites and the inter-dendritic spaces will bring about the fusion of the steel preferentially in these inter-dendritic spaces. During the operation of shaping this intergrowth of liquid and solid, the liquid phase will be ejected in a first stage at the start of the application of force. Therefore it is necessary to deform the solid phase and a residue of liquid for the most separated from the solid phase, which will result in an increase in the forces. For a deformation operation under these conditions the result obtained is poor: substantial segregation, internal defects.
On the other hand, when the thixoforging is carried out on a steel of globular structure brought to the semi-solid state by heating at a temperature between the liquidus and the solidus, the globular solid particles are distributed uniformly in the liquid phase. By optimising the choice of the solid/liquid proportions, it is possible to obtain a material having a raised rate of deformation under the effect of a considerable shear stress. It therefore has a very high deformability.
However, it is possible in certain cases to obtain the desired globular structure in the course of heating prior to the thixoforging, without having to carry out an operation of globulisation of the separated primary structure. This is the case in particular when operating on billets produced from rolled bars derived from continuous casting blooms or ingots. The multiple reheating and substantial deformations undergone by the steel have then led to a very imbricate and diffuse structure where a primary structure is practically impossible to show. It makes it possible to obtain a globular structure of the solid phase during the heating prior to thixoforging.
The thixoforging makes it possible, by comparison with conventional hot-forging processes, to produce in one single deformation operation parts of complex geometry which may have thin walls (1 mm or less) with very low shaping forces. In fact, under the action of external forces steels suitable for a thixoforging operation behave like viscous fluids.
For steels for mechanical construction, in which the carbon content can vary from 0.2% to 1.1%, the heating temperature necessary for the deformation by the thixoforging process is for example 1430° C.+50° C.=1480° C. (measured solidus temperature+50° C. to obtain the good ratio of liquid phase to solid phase necessary for the deformation) and 1315° C.+50° C.=1365° C. for a grade 100Cr6.
The heating temperature and the quantity of liquid phase formed are important parameters of the thixoforging process. The ease of obtaining the “good” temperature and the range of dispersion about this temperature so as to limit the variations of the quantity of liquid phase depend upon the solidification range. The greater this range is the easier it is to regulate the heating parameters.
For example, this solidification range is 110° C. for a grade C38 and 172° C. for the grade 100Cr6. Therefore it is much easier to work with this latter grade which has a low solidus temperature: 1315° C.
The very high shaping temperatures, the substantial rates of deformation which are used in the thixoforging process, lead to thermal stress on the deformation tools under conditions which are frequently extreme. This leads to the use for these tools of alloys with very high mechanical characteristics when hot or of ceramic materials. The difficulties of producing certain geometries or tools (inserts) of substantial volumes and the costs of producing them can slow down the development of the thixoforging process.
The object of the invention is to propose new grades of steel which are better adapted to thixoforging than those which are used conventionally in that they would make it possible to lower the shaping temperature and therefore to have less thermal stress on the deformation tools, and in that they would improve the behaviour of the steel during thixoforging. Moreover, these new grades should not degrade the mechanical properties of the parts obtained.